This invention relates to the need for alleviating the growing environmental problem of excessive plastic waste that makes up an increasing volume fraction of materials in landfills. Biodegradable polymers and products formed from biodegradable polymers are becoming increasingly important in view of the desire to reduce the volume of solid waste materials generated by consumers each year. The invention further relates to the need for developing new plastic materials that can be used in applications where biodegradability, compostability or biocompatibility, are among primary desirable features of such applications. There have been many attempts to make degradable articles. However, because of costs, the difficulty in processing, and end-use properties, there has been little commercial success. Many compositions that have excellent degradability have only limited processability. Conversely, compositions which are more easily processable have reduced degradability.
Useful fibers with excellent degradability for nonwoven articles are difficult to produce relative to films and laminates. This is because the material and processing characteristics for fibers are much more stringent, i.e., the processing time is typically much shorter and flow characteristics are more demanding on the material's physical and rheological characteristics. The local strain and shear rates are much greater in fiber production than other processes. Additionally, a homogeneous melt is required for fiber spinning. For spinning very fine fibers, small defects, slight inconsistencies, or non-homogeneity in the melt are not acceptable for a commercially viable process. The more attenuated the fibers, the more critical the processing conditions and selection of materials. New materials would ideally need to exhibit many of the physical characteristics of conventional polyolefins. They must be water impermeable, tough, strong, yet soft, flexible, rattle-free, cost-effective, and must be capable of being produced on standard polymer processing equipment in order to be affordable.
To produce fibers that have more acceptable processability and end-use properties, choosing acceptable degradable polymers is challenging. The degradable polymers must have good spinning properties and a suitable melting temperature. The melting temperature must be high enough for end-use stability to prevent shrinkage or melting. These requirements make selection of a degradable polymer to produce fibers very difficult.
Polyhydroxyalkanoates (PHAs) are generally semicrystalline, thermoplastic polyester compounds that can either be produced by synthetic methods or by a variety of microorganisms, such as bacteria or algae. The latter typically produce optically pure materials. Traditionally known bacterial PHAs include isotactic poly(3-hydroxybutyrate), or PHB, the high-melting, highly crystalline, very fragile/brittle, homopolymer of hydroxybutyric acid, and isotactic poly(3-hydroxybutyrate-co-valerate), or PHBV, the somewhat lower crystallinity and lower melting copolymer that nonetheless suffers the same drawbacks of high crystallinity and fragility/brittleness. PHBV copolymers are described in Holmes, et al. U.S. Pat. Nos. 4,393,167 and 4,477,654; and until recently were commercially available from Monsanto under the trade name BIOPOL. Their ability to biodegrade readily in the presence of microorganisms has been demonstrated in numerous instances. These two types of PHAs however are known to be fragile polymers which tend to exhibit brittle fracture and/or tear easily under mechanical constraint. Their processability is also quite problematic, since their high melting point requires processing temperatures that contribute to their extensive thermal degradation while in the melt. Finally, their rate of crystallization is noticeably slower than traditional commercial polymers, making their processing very difficult or cost-prohibitive on existing converting equipment.
Other known PHAs are the so-called long side-chain PHAs, or isotactic polyhydroxyoctanoates (PHOs). These, unlike PHB or PHBV, are virtually amorphous owing to the recurring pentyl and higher alkyl side-chains that are regularly spaced along the backbone. When present, their crystalline fraction however has a very low melting point as well as an extremely slow crystallization rate. For example, Gagnon, et al. in Macromolecules, 25, 3723-3728 (1992), incorporated herein by reference, shows that the melting temperature is around 61° C. and that it takes about 3 weeks to reach the maximum extent of crystallization at its optimal crystallization temperature.
Further poly(3-hydroxyalkanoate) copolymer compositions have been disclosed by Kaneka (U.S. Pat. No. 5,292,860) and Procter & Gamble (U.S. Pat. Nos. 5,498,692; 5,536,564; 5,602,227; 5,685,756). All describe various approaches of tailoring the crystallinity and melting point of PHAs to any desirable lower value than in the high-crystallinity PHB or PHBV by randomly incorporating controlled amounts of “defects” along the backbone that partially impede the crystallization process. Such “defects” are either branches of different types (3-hydroxyhexanoate and higher) or shorter (3HP, 3hydroxypropionate) or longer (4HB, 4-hydroxybutyrate) linear aliphatic flexible spacers. The results are semicrystalline copolymer structures that can be tailored to melt in the typical use range between 80° C. and 150° C. and that are less susceptible to thermal degradation during processing. In addition, the biodegradation rate of these copolymers is higher as a result of their lower crystallinity and the greater susceptibility to microorganisms. Yet, whereas the mechanical properties and melt handling conditions of such copolymers are generally improved over that of PHB or PHBV, their rate of crystallization is characteristically slow, often slower than PHB and PHBV.
In general, however, it has been a considerable challenge to convert these newer PHA copolymers, as well as other biodegradable polymers, into useful forms by conventional melt methods, for they remain substantially tacky after they are cooled down from the melt, and remain as such until sufficient crystallinity sets in, particularly with PHA copolymers levels above 10 wt %. Residual tack typically can lead to material sticking to itself or to the processing equipment, or both, and thereby can restrict the speed at which a polymeric product is produced or prevent the product from being collected in a form of suitable quality. Consequently, there is a need for an inexpensive and melt processable composition of degradable polymers. Moreover, the polymer composition should be suitable for use in conventional processing equipment. There is also a need for disposable articles containing nonwoven webs made from these fibers.